The present invention relates to resonators. More particularly, the present invention relates to a thin-film bulk acoustic wave resonator.
Resonators based on membranes or thin film layers are used to convert sound waves to electric signals, and vice versa. An arriving sound wave exerts a stress on the membrane, straining the membrane. Such resonators are made by depositing the film layers oh a substrate (usually SiO2) by various techniques including sputtering, and include a piezoelectric layer (piezolayer) sandwiched between a top electrode, the layer farthest from the substrate, and a bottom electrode, nearer the substrate. When the membrane (and hence the piezolayer) strain, an electric field is created in the piezolayer. The electric field is sensed as a voltage across the top and bottom electrodes, a voltage correlated to the sound wave. For example, in an article entitled xe2x80x9cAcoustic Bulk Wave Composite Resonatorsxe2x80x9d, Applied Physics Lett., Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is comprised of a thin film piezoelectric layer of Zinc-Oxide (ZnO) sputtered over a thin membrane of Silicon (Si). The resonator exhibits high acoustic reflectivity characteristics at interfaces between the air and device, therefore enabling the device to have a suitable figure of merit (Q).
To isolate the substrate from the acoustic energy created by the vibrating piezolayer, the prior art teaches using so-called acoustic mirrors, i.e. layers of materials presenting to arriving sound energy an impedance mismatch at the interfaces of the materials. An example of a resonator including an acoustic mirror is disclosed in the article entitled xe2x80x9cUltrasonics in Integrated Electronicsxe2x80x9d, Proc. IEEE, Vol. 53, October 1965, pp. 1305-1309, by W. E. Newell. The acoustic mirror in such a resonator may include a lower layer having a low acoustic impedance and a thickness of one-quarter wavelength, and an upper layer having a high acoustic impedance and a high reflectivity characteristic. In such a device, the lower layer functions as an xe2x80x9cimpedance converter,xe2x80x9d since it can transform the acoustic impedance of a substrate to a very low value. In a device where each of the layers has a thickness of one-quarter wavelength, the conversion factor of the pair of layers is equal to the square of the ratio of their respective impedances.
A voltage across the piezolayer, applied via the top and bottom electrodes, creates a mechanical stress and, consequently, an acoustic wave in the FBAR structure. The wave reflects back from the acoustic mirror and from the top air interface. The properties (amplitude and phase) of this wave are modified by the mechanical properties of the thin film stack. The sound oscillates in the thin film stack. At a certain frequency (called the series resonance frequency), there is a resonance condition and the amplitude of the wave becomes large.
The piezoelectricity of the piezolayer also works (transduces) in the opposite direction; a mechanical (acoustic) wave creates an electric voltage across the piezolayer. The mechanical energy to electrical energy transducing allows measuring the acoustic response of a resonator by means of an electrical measurement. In practice, the measured electrical quantity is the electrical impedance (ratio of voltage to current in the piezolayer). Because of the frequency dependence of the electrical impedance (which correlates to the mechanical or acoustic impedance), an FBAR structure is useful as a component of an electrical filter.
Unfortunately, because many layers need to be formed to create these types of devices, it can be difficult to form the layers to have precise xe2x80x9cdesignxe2x80x9d thicknesses. Also, during the fabrication of these resonators the process of sputtering the layers often results in the layers having incorrect thicknesses.
A further problem with these types of resonator is associated with intrinsic stress, compressive or tensile stress on a thin film layer created in depositing it on the substrate, i.e. stress on the thin film caused by the deposition process itself. The intrinsic stresses on the layer materials forming the resonators can inevitably strain the lower stack layers, eventually resulting in at least one of these layers being peeled from the substrate. This peeling problem becomes more severe for resonators having thicker layer stacks.
U.S. Pat. No. 5,873,154 to Ylilammi et al. teaches addressing both of the above problems, the thickness problem and the peeling problem, by using one or more low acoustic impedance polymer layers (of e.g. polyimide, so as to be able to withstand the high temperatures used in fabricating the resonator), and using high atomic weight, high acoustic impedance materials, preferably tungsten (W), for each electrode.
Besides the peeling problem associated with (substantially static) intrinsic stress, there is a mechanical strength/peeling problem associated with the high-frequency stress caused by an acoustic wave impinging on the resonator (or being generated by the resonator), a problem that is not addressed by U.S. Pat. No. 5,873,154 to Ylilammi et al. Because of a tendency for the layers to peel as a result of such high-frequency stresses, there is a limitation on the acoustic power that can be generated or received by a thin film resonator. The maximum power (i.e. the power rating) of a resonator is limited both by the mechanical stresses it can endure, as well as by the heat it generates in transducing acoustic energy to electric energy. The prior art teaches proper cooling of a resonator so that mechanical stress, not excessive heating during operation, is the primary limitation on the power rating. The mechanical stresses that can be withstood by a thin film resonator obviously depend on the stack construction, i.e. the particular materials used for the various layers of the resonator and the method of assembling the layers.
What is needed is a stack construction for a thin film resonator with improved capability to withstand mechanical stresses, giving the thin film resonator a higher power rating (when conventional cooling techniques are used in the operation of the resonator).
Accordingly, the present invention provides a filter and a method for fabricating a filter comprising a thin film bulk acoustic wave resonator (FBAR), the FBAR including a plurality of layers of different materials deposited on top of each other and on top of a substrate, the FBAR including a piezolayer sandwiched between a top electrode on the side of the. piezolayer facing away from the substrate, and a bottom electrode on the side of the piezolayer facing the substrate, the FBAR further including an acoustic mirror made from a number of layers of different materials selected to provide in combination high reflectivity of sound energy, the method comprising the step of forming the bottom electrode from a material having a small acoustic impedance.
In a further aspect of the invention, the bottom electrode is made from a material having a specific acoustic impedance of at most approximately 50 Gg/m2s.
In another, further aspect of the invention, the substrate is made from material selected from. the group consisting of glass, silicon dioxide, and gallium arsenide.
In yet even another, further aspect of the invention, the filter is fabricated to be a band-pass type filter.
A filter fabricated according to the method of the present invention is advantageously used as part of a mobile phone, in the transmitter/receiver section of the mobile phone, the transmitter section including a power amplifier used to amplify signals before transmitting the signals via an antenna, and in particular as a component of a transmitter filter, coupling the power amplifier of the transmitter section to the antenna.